Geophysical exploration system



1m? H, 1942. H. HQ OVER, JR., ETAL. 2,305,383

GEOPHYSICAL EXPLORATION SYSTEM Original Filed Sept. 30, 1938 5 Sheets-Sheet 1 8 l l l I I I 'I I l m i A w v JN'l/EN TO R8 HERBERT HOOVER JR.

, O C'HAkLES an. Mensa: L a l B7 l 70 1, 43 1/1 44; 1Y5; l kg mm Arrodlsvs TI 1, 1942. H. HOOVER, JR., ETAL 2,305,333

GEOPHYSICAL EXPLORATION SYSTEM Original Filed Sept. '30, 1938 5 Sheets-Sheet 4 of o m m m 6% 5 o m 0 5' 52 o o o o 5 on o a m w o m o m X; m m m m m m o X a 0 o m 0 m I 1 7 5, a 8 /S l 0 m 54 m m m? m w 5 0 m '0 o o 0 a a o i X X X m X 2 4 am w m 0 M? ml? INVEN raps HERBEAE'T HOOVER JE. CHAEL8 GILL MOEGAN NORMAN J. CHE/ST/E ATTORNEYS ateute cc. 5, l

Corporation, =1. Cornia Uriginal application how 3t, 19%, M w

c. ssignors to Coll e (3-, acomm w W No. 232,534.. Dividw March 19, 1940, Se No. $241,822

Our invention relates to geophysical prospecting, and more particularly to an improved apparatus for continuous subsurface exploration.

In conducting seismic exploration in California and many other areas, it has long been maintained that continuous subsurface exploration could not be done successfully, or when successful, it was not economical. As a result, geophysicists in these areas have resorted to the use of the less accurate and thereby less reliable and t application in 5, (Ci.

we have overcome the obstacles which have heretofore prevented the successful and economical application of continuous subsurface exploration in such areas dificult to survey.

The principal object of our invention, therefore, is to provide an apparatus of continuous subsurface exploration which will operate suc system of seismic exploration known as "dip shooting.

The failure of continuous subsurface surveying in many areas, such as some found in California, has been ascribed to the irregular character of the sedimentary deposits, the existence of accentuated lensing and pinching out of beds and the fact that the lithologic character of the beds varies greatly from point to point in a lateral direction. The lateral changes in lithology may be due to variations in the mineral constituents or to physical conditions such as cementation, moisture content, compaction et al., thus cessfully in dificult as well as simple areas.

Another object of our invention is to provide a system of continuous subsurface exploration in which it is possible to follow reflections from beds within a given formation from trace to trace in spite of change of character of the beds or the waves.

Another object of our invention is to provide a system for overlapping waves received at the surface by diflferent receptors, in such a manner that ground roll, consisting of surface or causing marked changes in the elastic properties of a given bed throughout a given area. In the Mid-Continent, there is such high contrast between the elastic properties of many of the strata that reflected waves may be properly correlated even though they be recorded miles apart without intermediate tie. But the vertical changes inthe elastic properties of deep subterranean depositsin many areas, as in California, are com-,

paratively small so that even small lateral changes in the stratigraphy have a profound ef-* feet on the character of the reflected waves. Be-' cause of the comparatively rapid =variations in the thickness and physical nature of the beds from point to point beneath a line of exploration in such an area, the character of the seismic waves reflected from a given underlying stratum also varies greatly. a

other reason for the failure of continuous L waves, is substantially reduced in amplitude comparedwith other waves.

Another objectof our inventionis to provide a system for obtaining continuous subsurface surveying and three-dimensional control simulta neously, thereby making possible the accurate determination of strike and dip of the horizon being identified from record to record.

another object of our invention is to provide a system for the reduction of ground roll without introducing electrical overlapping, which may produce undesirable cross-feed of energy between recording channels unless properly applied, as in co-pe'nding applications of Herbert Hoover, Jr., for United States Letters Patent on improvements in Geophysical prospecting receptor circuits," Serial No. 202,752, filed April 18, 1938, now Patent No. 2,266,041, of December 16, 1941, and on Geophysical prospecting receptor circui Serial No. 164,100, filed September 16, 1937, now Patent No. 2,266,0t0- of December 16, 1941.

Referring to the drawings:

Figs. 1, 2, 3 and a are schematic plan views of various pattern modifications of one form of our invention.

ins.

Fig. 7 is a pattern diagram showing a pre- Figs. 8, 9, 1o, 11 and 12 illustrate various other patterns our invention may take, with most of the receivers in the same line with the generating station.

;Figs. 13, 14, 15 and 16 illustrate patterns of our invention, with all the receiving stations in one straight line parallel to a straight line of generating stations.

Figs. 17, 18, 19, 20, 21 and 22 illustrate several of our patterns which may be used for obtaining three-dimensional control simultaneously with continuous subsurface exploration.

In the patterns illustrated receptor gravity centers are indicated by dots, and the surface projection of a generating point by, an X. It is to be understood that the actual shot points are preferabLv below the weathered layer of the ground.

In general, geophysical exploration is conducted by establishing generating stations and receiving stations, producing physical disturbances in the earth at the generating stations, and recording the cheats that such disturbances produce at the receiving stations.

In the most efiective method of geophysical exploration, use is customarily made of earth disturbances in the form of artificially created seismic waves. Usually these seismic waves are I. created by detonating a single charge of explosive below the weathered layer of the ground.

seismic or acoustic waves, and may be conveniently designated herein by the letter S, together with a subscript determining its position in a pattern.

In the seismic method the receivers, or receptors, (hereinafter sometimes referred to as receiving stations), used to detect the disturbances may be ofthe form of seismometers, geophones, acoustic receivers, or any other suitable vibration pickup. As regularly practiced, electrical waves converted from arriving earth waves at the receivers are amplified electrically, transmitted, and recorded as oscillograph traces on photographic paper, usually several on a single strip. Frequently, the outputs from several receptors are combined in some manner before the recording. The point in space representing the center of gravity of the places where energy translation from ground to apparatus occurs in the receptor is designated by the letter R, etc.

It is to be understood that the patterns herein to be described are based on a fundamental concept of a hypothetical planar subsurface stratum. However, all patterns laid out on the surface of the ground over unknown subsurface structures before knowledge thereof will. when the actual conditions do not coincide with the hypothetical premises, give results corresponding with the actual conditions, thus allowing the survey to be accomplished.

Other objects of our invention will be apparent or will be specifically pointed out in the description forming a part of this specification, w

Inthisanbutwe do not limit ourselves to the embodiment of the invention herein described, as various forms may be adapted within the scope of the claims Fig 6 shows a typical record obtained in easy shmting territory with a prior typeof seismic prospecting method. In this record the character of the waves changes but slightly from one receiving point to the next, so that the correlation of waves from trace totrace is very easy. A series of correlated waves is joined by a broken line M. In a widely used method of continuous subsurface exploration, a series of such records was made by using'a continuous series of spreads of receivers positioned on a line of shot points;

important of all is the fact that for the records obtained successively along the line, there emst wave-travel paths and reflection points which are so close to each other that the recowition of: waves from one record to the next is extremely simple.

By shooting continuously. in the manner described in the Westby article, important changes in dip and faulting may be determined.

When attempts are made to apply this prior method, however, to areas having beds of rapidly changing lithologic character, it is often dimcult and frequently impossible to trace a reflected wave from a subsurface bed from trace to trace when explorationis conducted according to the procedure outlined above. In many areas the character of the surface is such that the waves received at the receptors adjacent the shot-holes are completely masked by surface vibrations and noise from the generating point. Forthis reason the waves received at .a receptor at the shot point position are not easily identifiable, and hence there is a resultantlaci: of continuity in the identification of waves refiected'from the subsurface. In areas of rapidly changing lithologic character this may result in mistaking reflections from one bed on a first set-up for those from another bed on another set-up.

One of the dimcultiw with this prior method arises because the waves which should be received at receptors placed at the seismic wave generating points are frequently masked by the noise at such points, and because the characteristics of the soilat the generating point may be changed in the process of drilling a shot hole there or by virtue of the detonation of an explosive charge at the bottom of the shot hole.

In order to avoid such difficulties and thereby make continuous subsurface explorationpossible in such areas, in some forms of our iiivention we ofiset from the line of generating points the reception point that would otherwise be located at the generating points. In other form of our invention, instead of using single reception points ofiset from the respective generating points, we locate pairs of receiving stations substantially parallel to the line of generating points, with the midwint between'the receiving stationsofiset rticular earth material present.

assess generopposite the correspon a: point. In most forms of our invention the cheat reception points serve to link spreads of reception points located in the region between successive generating points. with such apparatus arrangements as are provided by our invention, equal wave travel paths may be achieved ior waves reflected from substantially the se point on the subsurface whenever diderent com b v ..tions of shot points and reception points are for recording waves reflected from successive portions on a reflecting horizon and the etection of such waves is assured. Methods for v a; the detection of such waves are described hereinbelow and claimed in our copending patent application, Serial No. 232,534, filed September 39, 1938, of which this application is a division. v

In 1 i we have illustrated how the undesirable edects oi shot-hole disturbances may be reduced by oflsetting the receptors adjacent the shot-hole from the main line of receptors and. connecting them with a chain of receptors, prei-' erably along the line Joining the shot-holes.

The amount of ofl'set required to produce records iree oi shot-hole disturbances varies greatly in a erent areas. As pointed out above, there are some areasin which the disturbances from the shot-hole are not great enough to prevent using a receiving point right atthe shot point 'or within a few Iet of it. In other areas, however, any receptor within as much as seventyfive or a hundred feet, or even more. is so much edected by disturbances from the wave generator that reflected waves cannot be identified on the record obtained.

In addition to avoiding shot-hole disturbances there is another important reason for not placing a receptor right above the shot. In y are we have observed that when a shot is fired in or below the weathered layer, the shattering oI the earth material causes a considerable reduction in the velocity of seismic waves theretime delays in the wave arrival by as much as iiveand sometimes twenty-thousandths of a secand, depen on the size of the charge and the This edect tapers ed with distance from the shot-hole. In some cases we have observed that the receptors must be odset from the shot-hole as much as 25 or 50 feet in order to make errors from this source neg 1 characteristic may he termed the corwa eflect." In Fig. 2 is shown a system similar to that shown in Fig. l, but a plurality of receptors is used in place oi =N:- 1e receptors. In this case the pllity of receptors is illustrated by a w I rs oi each pair being placed apart apprately half a wave length of the ground D g well known principles of wave inter- Ierence.

ible. For brevity this phenomenon of velocity change resulting from such changes in said present they may both be reduccd by .suriace t1 In that of Fig. 2. however, ground waves arriving at a pair of receptors, say Rs, from a distant shot-hole, say 83, do not cancel out. I 7

In many areas the ground wave may be so bed in amplitude over such a distance that it will not mask the reflected waves. In other cases, however, the ground wave from a given shot-hole will still have large amplitude at the neighboring shot-holes. This difficulty has been overcome by arranging a group of receptors at the odset receiver position in such a manner that it is substantially insensitive to ground roll from either of two diflerent directions. This is illustrated, for example, by the two ements of Figs. 3 and 4.

In B. 3 and i we have illustrated, by way of example, arrangements of four receptors offset from the shot-holes in such a way that substantial cancellation of the ground wave will be produced for two approximately perpendicular directions. With these arrangements ground waves received at receptor position R9 iromshots at 81, B2 or So are considerably reduced. The spacing of thwe receptors is made in accordance with well mown principles of wave interference. If desired, the receptor groups in Figs. 2 to 4 may i 1.1 larger numbers of receptors arranged to produce substantial cancellation of ground roll.

In many cases oflsetting the receptor or re ceptor group alone is suflicient to provide continuous subsurface coverage; By so offsetting the receptors the non-recognition of common recordings ior set-ups of adjacent shot-holes and reflected from substantially common incidence points on the reflecting bed is obviated, and there is less danger of mistaking reflections from one bed for those from another. Frequently, however, the spacing between adjacent receptors or receptor groups is too great for the positive identification of waves which are changing in character irom point to point, especially where this ge in character is rapid. In order to overcome thislatter dimculty we have increased the number of receptors and decreased the discc between the receptor groups. In many.

= we have also diminished the distance between shot-holes. to some extent. Heretofore shot-holes have ordinarily been placed from a.

hair to a mile apart, but this is not satisfactory in very complex territory.

In some cases we have found that satisfactory results can be obtained economically by spacing the shot points as little as 500 feet apart and -r 1' the distance between receiving positions to as low as 30 feet. In an ideal system for use in extremely complex territory, it would be desirable to increase the number oi receiving itions indefinitely so that they are Just a few feet .or even a few inches apart.- However, this is not a practical or economical solution of the problem. Heretoiore, the minimum receptor interval used in continuous subsurface exploration has been about 120 feet. This of course is satis factory in the Mid-Continent, and some parts at r d other easy shooting regions. Reted atpts to use this latter distance in r h or California and other complex terthat even in dip shooting it was necessary to plant the receptors closer mether, but it was considered impractical and uneconomical to use this closer receptor planting in continuous sub- I loration.

Up to the time of our invention the largest number of chels in one recording unit in oi receptors may seismicexploration was twelve. It was considered uneconomical to use more. The result was that continuous subsurface exploration in California-type areas was uneconomical and never practiced.

By increasing the number of recording channels and thereby making possible reduction of the receptor interval, we found it was not only possible but also economical to conduct continuous subsurface exploration in such areas.

In order to further increase the recognizability of reflections from trace to trace, we have adopted a system of spatial overlap. To understand this part of our invention consider a line of receptors R1, Rs in Fig. 5. The outputs from receptors R1 and R4 are combined and applied to a single amplifier amp. I. Similarly, the inputs for amplifiers amp. 2, amp. 3, amp. d, etc., are obtained by combining outputs of receptors as shown. In this way an efiect that is more a or less an average over a large region is recorded on a single channel so that rapid lateral changes in lithologic character are averaged out and the recognition of waves from the same horizon from trace to trace is greatly facilitated. If desired, sets ofa larger number of receptors can be planted in any suitable manner to provide substantial overlap of the subsurface areas from which the reflections arise and the outputs from the receptors in each set combined, and the. combined 1 output for each set applied to a separate record-- ing channel.

In pursuing our invention, shots may be made for recording at receptors on one of both sides of the shot point. For convenience we may refer tions lie on the other side.

For example, using the patterns of Figs. 1 to 4, we may shoot as follows:

Method 1 Bece tors at whic waves are recorded simultaneously Shot at shot point Rn Rn R11 {R11 R9 rammar anon- Receptors at Shot at shot which waves 1min simultaneously i f S2 1 etc.

"In the first series of shots using unilateral set-ups, two shots are taken at each shot point except those on the end. If desired, a spread be planted outside the line oi in a spread lie substantially on one side of the "generating station and one or more receiving staare recorded asoaeas shots and two shots taken at the end shot points as well.

In the second method a bilateral pattern is used so that only one shot is taken per shot point, in this manner greatly improving the accuracy of the data by helping to insure positive identification of waves from a given bed received on 01)- posite sides of the generating stations. This improvement in result originates from the fact that the wave-form from one shot to the next, even in the same hole, varies to at least some extent, as is'well known; by shooting bilaterally this variation from shot to shot in the same hole is eliminated. From the travel times of the waves for the shot points to the associated receptors the depth and dip of the formation. may be determined byxwell known methods. By using such a process there will always be a trace on each record of substantially the same travel time,,and reflected from very near the same point as for a trace on a record taken adjacent to it.

Sometimes it is necessary to join the oflset receptors with the receptors in the shot-hole line by one or more addition receptors. Ifthis is not done, it inay be very difllcult to identify the proper wave on the traces obtained from the offset receptors. One simple way to accomplish the desired result is shown in Fig 8.

In order to carry out our invention it is not necessary to place the offset receptors on a line perpendicular to the line of exploration at the generating points. We may, if we wish, lay out a line of shot-holes and place a line of receiving stationsbetween each successive pair of shotholes, as in Figs. 10, 11 and 12. These lines of receiving positions intermediate the shot-holes terminate in receiving positions far enough from the shot-holes so as not to be disturbed by ground vibration or noise from the shot-hole. For convenience we shall call such a spread an intermediate line of receiving stations". Then two receiver positions. such as the end receiver position as R11 and R14 of each line on the two sides of a shot-hole S: of Figs. 11 and 12. are joined by a chain of receptors spaced closely enough together to permit recognizing waves reflected from a given bed from trace to trace. Each receiver position in the chain, however, is placedfar enough from the shot-hole so that noise and other undesirable vibration from the shot-hole 'will not disturb receptors placed at such positions;

In adopting such-patterns we may vary the spread used in many ways. Considering one shot point alone, as for example S2 in Figs. 11 and 12, we may record waves from a single shot received at anyone of the following group of receiving stations: R1 RisorRz R14; R: R13; R4...R12; R1...Ra4orRz...R23; Ra...R22; and variations of these patterns. Considering" just one pattern, suppose waves received at Rs Rza are recorded for a shot from S2, and waves received at Ru R32 are recorded for a, shot at $3. In this case there is actual overlap of the incidence points on the reflecting horizon.

This arrangement has the additional advantage. that if there is any peculiar discontinuity in this overlapped region this anomaly will appear on two records. Any irregularity at incidence points corresponding to the tie-in receiving positions as Rs in Figs. 1, 2, 3 and a will not impair the results, as there are overlapping incidence points to provide a complete check .on the data. The time that would be occupied for a wave to travel from S: to the reflecting horizon and to a receiver to one of folio prov a, 1-: vations therefrom:

etliwd a Recs tors at whi waves are ed simultaneously Shot at shot point Receptors at Shot at shot which point llfet 5 Recs tors at whi waves are recorded simultaneously shot at shot Set-up point 11s.. Run- Same at.

Rose tore at whi Waves are remrded simultaneously Shot at shot point Sn R; R21! Bu R1: Rn etc.

a thetical flat subsurface or in such a 1. -er that the chains of incidence points corresponding to each shot-hole receiving stations so to form a continuous c of receivers, then generating waves at sucve stations and recording the waves received at suitable groups of receiving stations so as to provide continuous coverage of the underlying strata. In the preferred embodiments of this form of our invention the intermte lines of receiving stations lie wholly on the line of generating stations.

Satisfactory results may be ob by shooting at given geneg station and recording at an acent spr and. shooting again at the e position and reco at an adjacent spread on the other side, d repeating the prmess. at other generating'stationndown the line in such a overlaps the c of inductance points from a n w i1 shot-hole.

vantage in placing receiving stations on the line of shot-holes lies in the fact that in such a 1 a M ement it is very; easy to provide for cantion of ground roll as cancellation in two opposite 1 tions only is required. This may be done by using pairs of receivers as for the intermediate lines of receivers in Fi 2. Another advantage in plac the recei stations on the line of shot-holes lies in-the fact that in rough territory it is especially desirable to place the shotholes and receiving positions along the same line which very frequently is a road. In this way it is in s. 13, l4, l5 and 16. In these figures the receiving positions are placed in straight lines parallel to the line of generating stations. This type of set-up i especially convenient wherever local laws forbid the drilling of shot-holes near roads. W receptors may he placed beside the m and shot-holes odset therefrom. This overcomes the ri of cutting away lbrush,- etc. along an line of exploration for-planting receivers using these patterns. It is necessary to cut out o short surface paths to the generating points. as illustrated in Figs. 13 and it, re is a receiver station opposite each shothole. in s. 15 and it, the position opposite each t-hole separates two receiving positions.

:1 agements, recordsare made acpciples already discussed, using air one or two shots per generating station ac- 1 to whether or not unilateral, substantially w a or hilateral reads are used.

lin any of these patterns the receptors should he placed close enough together to permit the recotion of corresponding waves from trace to trace; in out areas they should be less .1 1 feet apart, and in very dimcult areas less than 50 feet part. ere needed, provision should he made for cancellation of ground roll in or more directions. Where lithologic cacter varies rapidly from point to point, spatial or electrical overlap may be utilized to advantage. v

The size of the gaphetween portions of the subsurface explored should preferably be equal to or' less than the distance between successive incidence points within the same record. However, when the wave-form varies but slightly from trace to trace on a givenrecord, as frequently occurs in easy shooting territory, the gap may be larger.

The computation of the dip and depth of the reflecting beds may be made in several ways. They may be made from the travel times to the receptors in the odset receiver positions, or they be made from the travel times to the receptors at the end and receivingpositions' along the shot line.

The required computations of weathering corrections may be made by principles well known to those smiled in the art. 1

By applying our method for obtaining positive correlation from trace to trace and record to a record, waves reflected'from a given bed may be followed alonsya line of exploration, and by appropriate arithmetic computations the difference of the dip of that portion of ,for various shot points down the line.

Wherever the cross-line component'of dip (that is, the component of dip in a plane perpendicular to the'wavatravel plane) is small, or constant from one set-up to another, the relative depths of a reflecting horizon can be computed at various points along the line and valuable information obtained therefrom. However, the crossline component of dip is not always so small that item be neglectedin the computation of depth. Whenever the cross-line component of dip difiers at two shot pointsythese values must be known in order to determine an'accurate difference in the depth of the reflecting horizon at the two points. Otherwise, erroneous results are obtained and invalid conclusions drawn. In California-type territories it is necessary to keep accurate information of the cross-line component of dip, as in such territories the dip is often high and the strike often changes rapidly along the line of exploration. Only in this way can accurate subsurface contouring be provided.

In order to make possible the accurate subsurface contouring of complex strata, we have modified our invention to provide accurate information of both the in-line and the cross-line components of dipsimultaneously while carrying out continuous subsurface surveying.- Some of these embodiments of. our invention are illustrated in Figs. 7 and 1'7 to 22.

In Fig. 1'1 we have illustrated patterns in which there are shown continuous chains of receiving stations close enough together to provide positive identification of wavesfrom trace to trace, and yet none of the receivers is so close to the generating station as to be seriously disturbed thereby, or so close for the corona efiect to produce errors in the weathering corrections. If desired or necessary, a suitable chain of receivers may join R1, R25, Rs and R20. This pattern may be used ina variety of ways. procedure, that is, the one most accurate, recordings are made at a cross of receivers for each shot point. Shooting from S1 we record at the receivers R22 to Rio, as well as in accordance with the procedure for those receiving positions of Fig. 17 forming a pattern similar to those shown in Figs. 1 to 4, and 8 to 12.

A similar shot is taken at $2.

As a result of this procedure, we get positive identification of waves from the same bed at all the receiving stations so that we are able to compute both the in-line and cross-line components the bed returnin waves to each receiving station. For the setup for S1 we can compute the T in-llne component of dip from the difierence in travel times for the reflected wave at R1 and R11; and the cross-line component of dip may be computed from the difference in arrival times at R211 and R29. In such computations, appropriate account is of course taken of elevation corrections, weathering corrections et al.-

By our procedure we are thus able to compute the position of the reflecting horizon accurately and thereby poses to using obtain a more complete and satisfactory geophysical picture than has been obtained heretofore.

In Fig. 18 is shown another embodiment of our invention, in which we obtain both in-line and cross-line components of dip simultaneously with conducting continuous subsurface exploration. Recording simultaneously at R1 to R13, R21 to R25, and R26 to Rae for waves generated at $1, and repeating the process similarly for other shot points down the line, we obtain the records desired. Using the time difference in arrivals at R1 and Ru, and the difierences in arrivals at B1 and R15, and also the difference in arrivals at R10 and Ran the dip components may be computed and the position of the reflecting bed accurately located in space.

Using this technique is superior for certain purthat described with Fig. 17. In the procedure suggested for use with Fig. 1'1 the cross-line component of dip of the portion of the bed reflecting waves to the spread extending 7 across the line of exploration is obtained. Using In the most suitable I rapidly from point to point.

the procedure suggested with Fig. 18, the crossline component of dip at two parts of the reflecting horizon'for each set-up is obtained. For the shot from S2 we get the value of cross-line component of dip corresponding to the line of receptors R4, R21 R25, and another value of the cross-line component of dip corresponding to the line of receptors R10, Rae Ban. This results in still more detailed information of subsurface con ditions.

Figs. 19, 20 and 21 are further illustrations of continuous lines of receptors that may be used in connection with our invention. .Though the patterns shown in Figs. 19 and 20 provide substantially continuous subsurface coverage only if the In Fig. 21 we have illustrated a preferredpattern to be used with our method to obtain data for accurate subsurface contouring inhighly complex territory. By weaving the chain of receptors back and forth across the line of exploration, as illustrated by this example, it is possible to determine changes in both in-line and cross-line components of dip within the portion of the subsurface that reflects waves to any chain of receptors recording the waves from any generating point.

.Figs. 7 and 22 illustrate still further modifications of our method for determining subsurface contours. In Fig. 22 the receiving stations R1. R11, R1: and R22 are placed near the generatin stations but far enough therefrom to avoid the "corona effect." R1 and R11 are connected by a continuous chain of receivers; R12 and R22 by a' a then the waves arriving at the receiving stations R12 R11 R22. It is clear that -a reflected wave generated at generating station S: and received at receptor station R11 will be reflected from substantially the same point on a reflecting horizon as a reflected wave generated at generate ing stations: and received at receptor station R12. It is also clear that these two waves will transverse spreads simultaneously in order to assure obtaining the strike and dip of the horizon require substantially the same time to travel over their respective paths and that these paths will be of substantially the same length. Though this statement neglects the effect of differences in tions substantially parallel to each other, as in Fig. 14. Crossing said line of receiving stations near each generating station, but ofiset therefrom, there is a transverse line of receiving statlons. Recording waves from S2 at an adjacent parallel spread as R1 R13 and a transverse spread as R33 R44 simultaneously, and repeating the process for other generating stations along the line of exploration so as to obtain continuous identification of a subsurface bed, makes possible accurate subsurface contouring of said bed.

Now it is clear that the procedure to be used with any of the patterns of Figs. '7, and 17 to 22 may be varied in many ways. Records may be made unilaterally or bilaterally. Receivers may be placed opposite the shot points, or they may bracket points opposite the shot points, or placed in other suitable positions. The end reflecting points of the portion of the subsurface explored on each record may be identical with those from a record of waves from an adjacent portion of the subsurface, they may overlap, or they may fall just short of each other so that the portions of the reflecting horizon for successive set-ups have small gaps between them.

Groups of receivers may be placed at each receiving station to improve the type of record obtained. The distance from any receiver to the nearest generating station may vary over a wide range. Receiving stations may lie in straight, broken or curved lines. When applying our method to subsurface contouring, the value of the cross-line component of dip may be determined at various points on the line of exploration, or at every set-up.

The essential features of our invention are:

(a) The incidence points on the subsurface corresponding to traces on a given record shouldbe close enough together to assure positive correlation of waves from the same bed from trace to trace.

(b) The successive portions of the subsurface should be at least contiguous to each other and preferably overlap to some extent. Whenever said successive portions do not overlap they should preferably be no farther apart then incidence points on the portion reflecting Waves to one record.

(cl The travel times of the waves incident on a given bed in the contiguous or overlapping portions of a reflecting horizon should be substantially the same.

(d) No receiver should be placed so close to the nearest generating station as to be disturbed Jynoise or heavy surface vibrations therefrom.

(c) When there is a possibility that the cross- .lne component of dip varies rapidly along the ine of exploration, records should be made on which is being traced from record to record.

(I) The transverse lines should be part of or linked to the main chain of receivers.

(a) For most economical use of our invention, any large number of receivers may be used, the more difiicult the territory, the more complex the pattern, and hence the greater the number of receivers required.

1 For simplicity we have illustrated our invention schematically in order to simplify the drawings and 'the discussion. For simplicity in explaining our invention, it has been assumed that the reflecting bed was parallel to the surface. This was done for illustration only. Our invention can be applied to territories of any dip whatever. While it is true that special methods of computation must be used in cases of high dips of say 40 degrees or more, and still more complicated computations made where the strata are overturned, nevertheless our invention provides a simple and accurate method for obtaining the data required for such computations simultaneously with the identification of a given bed from one record to the next. The actual process of making such computations can be carried out by a person skilled in mathematics.

We claim: e

1. Apparatus for seismic surveying, comprising a series of spaced spreads of receiving stations, each station within each spread being spaced from other stations in said each spread, the entire series of spreads delineating a line of exploration, means for generating seismic waves positioned between end receiving stations of ad I jacent spreads, said end receiving stations of adjacent spreads being linked by an additional spread of receiving stations, said additional spread being outside of the line delineated by said prior recited generating and receiving stations, and' means for recording thewave disturbances received by the receiving stations in each spread.

2. Apparatus for seismic surveying, comprising a series of spaced spreads of receiving stations, each station within each spread being spaced from other stations in said each spread, the entire series. delineating a line of exploration, means for generating seismic waves between end receiving stations of adjacent spreads, end receiving stations of adjacent spreads being linked by at least one receiving station outside of the line delineated by said generating means and spreads of receiving stations, means for recording waves received by each receiving station in a spread, and-means for recording the waves received at said additional receiving station as a trace linking the traces of end receiving stations in adjacent spreads.

3. Apparatus for seismic surveying, comprising a plurality of seismic wave generating stations, a plurality of spreads of linked receiving stations positioned to receive waves from said generating stations along a line as delineated by the position of said generating stations, additional receiving stations positioned outside of said line, and means for snnultaneouslyrecording waves received by both in-line and out-of-line receiving stations.

4. Apparatus for seismic surveying, comprising a plurality of seismic wave generating stations, a plurality of spreads of linked receiving stations positioned to receive waves from said generating stations along a line as delineated by the position of said generating stations, additional receiving station spreads positioned outside of said line adjacent a generating station, and means for simultaneously recording waves received by receiving stations in both in-line and out-of-line spreads' 5. Apparatus for seismic surveying, comprising a plurality of seismic wave generating stations delineating a line of exploration, a plurality of seismic wave receiving stations positioned in a formation both progressing with and crossing the line of exploration, and means for simultaneously recording waves received by in-line and out-ofline receiving stations.

6. Apparatus for seismic surveying comprising a seismic wave generating station, two series of spaced seismic wave receiving stations, each of said series being spaced from said generating station and lying in a different direction therefrom and a chain of at least two seismic wave receiving stations spaced from said generating station lying outside the lines formed by each said series and joining said two series. of spaced seismic wave receiving stations, and means for recording seismic waves received at said seismic wave receiving stations.

7. Apparatus for seismic surveying comprising a seismic wave generating station, two series of spaced seismic wave receiving stations, each or said series being spaced from said generating station and in line therewith but in different directions therefrom, and at least one additional seismic wave receiving station spaced from said generating station joining said two series of spaced receiving stations in one continuous chain of receiving stations, and means for recording seismic waves received at said seismic wave receiving stations. l

8. Apparatus for seismic surveying comprising a seismic wave generating station, two series of spaced seismic wave receiving stations, each of said series being spaced from said generating station and in line therewith but in opposite directions therefrom, and at least oneadditional seismic wave receiving station spaced from said generating station joining said two series of spaced receiving stations in one continuous chain of receiving stations, and means for recording seismic waves produced at said generating stations and received at said seismic wave receiving stations.

9. Apparatusjor seismic prospecting comprising a line of generating stations, a line of seismic wave receiving stations substantially parallel to and spaced from said line of generating stations, and a plurality of lines of seismic receiving stations joined to said continuous line of receiving stations, each of said plurality of lines crossing the line of generating stations at a point intermediate two adjacent receiving stations, and means for simultaneously recording seismic waves received at one of said plurality oil-crossing lines and at a plurality of receiving stations on said continuous line all of said receiving stations at which waves are simultaneously recorded form- I ing a connected chain of receiving stations.

10. Apparatus for seismic surveying comprising a line of at least three seismic wave generating stations, 9, continuous line of seismic wave receiving stations extending substantially parallel to said line of generating stations and with the ends thereof opposite the end generating stations. and means for recording seismic waves produced at one of said generating stations and received at a plurality of said receiving stations.

11. Apparatus for seismic surveying comprising a continuous line of seismic wave receivin stations, a line of at least three seismic wave generating stations substantially parallel to said line of receiving stations, the projection of each of said generating stations on said line of receiving stations lying intermediate two adjacent receiving stations, and means for recording seismic waves produced at one of said generating stations and received at a plurality of said receiving stations.

12. Apparatus for seismic surveying comprising a continuous line of substantially uniformly spaced seismic wave receiving stations. a pair of seismic wave generating stations lying on a line parallel to said line of receiving stations, the

projection of each of said generating stations onto said first line substantially bisecting a line joining two adjacent receiving stations, and means for recording seismic waves produced at one of said generating stations and received at said receiving stations.

13. Apparatus for seismic prospecting comprising a plurality of seismic wave generating stations delineating a line of exploration, a plurality of seismic wave receiving stations positioned in a continuous lineal formation, part of which intersects said line of generating stations and part or which lies outside said line of generating stations, and means for simultaneously recording waves generated successively at said generating stations and received at related continuous groups of both in-line and out-of-line receiving stations.

14. In apparatus for seismic surveyi g the combination comprising a plurality of seismic wave generating stations delineating a line of exploration, a plurality of seismic wave receiving stations associated with each of said generating stations, each of said seismic wave receiving stations being spaced from each of said generating stations and a receiving station of each said plurality lying on a common reference line substantially parallel to the line of generating stations and substantially opposite a generating station next in line to said each generating station.

15. In. apparatus for seismic surveying, the combination comprising a. plurality of seismic wave generating stations delineating a line of exploration, and plurality of spreads of receiving stations, each of said receiving stations bein spaced from each said generating station and each of said spreads being symmetrical about a line passing through a corresponding generating station and perpendicular to said line of exploration, the end receivlng stations of each spread which are furthermost from the correspondin generating station lying substantially opposite another generating station.

16. In apparatus for seismic prospecting, the combination comprising a plurality of seismic wave generating stations delineating a line of exploration, a plurality of receiving stations lying on a. line substantially parallel to said first line, each of said receiving stations being substantially perpendicularly offset from a generatin station and a chain of closely spaced receiving stations joining altemate.receiving stations of said plurality of receiving stations.

1'7. Apparatus for seismic prospecting comprising an oblong rectangular configuration having a pair of seismic Wave generating stations at ends of one long side thereof, a pair of seismic wave receiving stations at ends of the other long side thereof, and a chain of seismic wave receiving stations joining said aforementioned receiving stations in one continuous chain, and means for simultaneously recording waves generated successively at each of said generating stations and received at said receiving stations.

18. Apparatus for seismic prospecting, comprising a plurality of seismic wave generating stations delineating a line of exploration, a plurality of continuously connected lines of seismic wave receiving stations positioned in a formation at least part of which lies outside the line of generating stations, and means for simultaneously recording waves successively generated at said generating stations and received at spreads of said receiving stations.

19. A geophysical exploration apparatus laid out in the following plan:

wherein S denotes a seismic wave generating station, R a seismic wave receiving station, and

wherein the subscripts of S and R denote the 10 position of S and R in their respective series.

(HERBERT HOOVER, JR.

CHARLES GILL MORGAN. NORMAN J. CHRISTIE. 

